The overall objective of the two proposed Center Collaborative Research Projects is to fill specific gaps in our knowledge of the impact of in utero, early postnatal and pubertal environmental exposures on mammary gland development at the cellular, molecular, organ and population levels and their influence on future breast cancer risk. This will be accomplished through 1) an epidemiological study to identify early exposures that can influence breast cancer risk through their effects on breast development in utero through puberty. This study takes advantage of a unique opportunity to follow-up a multiethnic cohort of 500 girls ages 4-11 for whom we have extensive in utero exposures and birth characteristics information and the opportunity to prospectively collect their childhood exposures that influence their hormonal milieu (estrogens, antiestrogens) and growth (diet, exercise, stress, insulin biomarkers), identify relevant genetic polymorphisms and relate them to the timing of pubertal development. 2) The second collaborative research project will use the mouse mammary gland model to determine the molecular mechanisms of mammary gland pubertal development and maturation through in vivo and in vitro studies on the mechanism of progesterone action focusing on the roles of PRA and PRB isoforms. In parallel with the epidemiological studies, the effects of in utero, lactational and postnatal exposures to environmental estrogens, antiestrogens and diet on mammary gland pubertal development and maturation will be determined. Finally, the effect of these environmental exposures on subsequent susceptibility to mammary tumor development will be determined. The overarching hypothesis is that in utero, lactational/formula feeding and post-lactational/childhood exposures to environmental stressors that affect the timing of pubertal development and sexual maturation impact on breast cancer risk. We specifically hypothesize that accelerated pubertal development and early onset of menarche increase breast cancer risk by increasing lifetime exposure to progesterone acting in concert with estrogen. To facilitate the inclusion of community concerns into Center Research Projects we will partner with community advocates and other stakeholders, and be responsive to the input from community-based, faith-based and advocacy organizations that are interested in breast cancer and environmental risks. To optimize translation of new findings into the most effective risk minimization, the Community Outreach and Translation Core will develop strategies and tools to transmit the information to populations of women and girls, as well as community stakeholders and policy-makers.